This invention relates to a security element for the electronic surveillance of articles.
Security elements in the form of soft magnetic strip material are frequently used in department stores for the protection of merchandise against pilferage. To accomplish a sufficiently high detection rate, sophisticated methods are employed which reduce the risk of false alarms effectively. Thus, it is known from EP 123 586 B to emit into the interrogation zone, in addition to two interrogation fields with the frequencies f1 and f2 in the kilohertz range, a field with a frequency in the hertz range. The two interrogation fields with the frequencies f1 and f2 excite a security element present in the interrogation zone to remit a characteristic signal with the intermodulation frequencies nxc2x7f1xc2x1mxc2x7f2 (where n, m=0,1,2, . . . ). The low-frequency interrogation field has the effect of driving the security element from saturation in one direction to saturation in the other direction at the clock rate of this particular field. As a result, the characteristic signal occurs cyclically with the frequency of the low-frequency field.
As an alternative solution of a system referred to as a harmonic detection system, it has further become known to use only one interrogation field in the kilohertz range for excitation of the security element, with the characteristic signal of the security element occurring again at the clock rate of a low-frequency field cycling the soft magnetic material between the two states of saturation.
For purposes of evaluation, the shape of the characteristic signal is subsequently compared with a predetermined signal shape. If the two coincide, this is interpreted as the unauthorized presence of a protected article in the interrogation zone; an alarm is then produced indicating to sales staff that a theft has occurred.
The hysteresis loops of security elements manufactured from a soft magnetic material follow an essentially linear course between the two states of saturation. In addition, security elements have become known whose hysteresis loops exhibit an abrupt transition between the two saturation states as soon as an external magnetic field compels them to change their saturation direction. Materials showing discontinuities between the two states of saturation are referred to as xe2x80x9cmaterials with Barkhausen effectxe2x80x9d. They are characterized by high permeability already in the low-frequency range, which means that they are excited to emit a characteristic signal by low-frequency interrogation fields. Further, the characteristic signals of Barkhausen materials feature a sharp peak. This again means that a relatively large signal component occurs in high-order harmonics.
Materials of this type are manufactured by subjecting soft magnetic materials to a suitable annealing process. The properties of these materials and the physical principles of the Barkhausen effect are described in detail in U.S. Pat. No. 4,660,025 and also in EP 0 448 114 A1. In particular, these prior-art specifications make reference to how soft magnetic material can be subjected to special physical treatment to turn into a material with high Barkhausen effect.
Security elements utilizing the Barkhausen effect differ significantly from soft magnetic security elements as regards their signal shape. As a direct consequence thereof, the probability of their being detected in so-called harmonic systems is relatively low soft magnetic security elements, in turn, are difficult to detect in systems designed for the detection of security elements utilizing the Barkhausen effect. The situation is different if it is aimed to have a security element detected in both a harmonic and a Barkhausen surveillance system. Where both types of security element, that is one employing soft magnetic and one employing Barkhausen material, are combined for integration into one security element, these may, under circumstances, influence each other through their magnetic fields such that the intermodulation signal of the combined security element is detectable neither in a harmonic nor in a Barkhausen surveillance system
Another prior art reference, U.S. Pat. No. 5,565,847 describes a tag which has a plurality of resonating sections each having separate frequencies of excitation. Each of the strips has the same material in different lengths in order to achieve different oscillating frequencies. This is to provide a multi-bit tag.
According to EP 0 295 085 A1, an anti-pilferage tag is disclosed which uses different ferromagnetic material compositions and in particular a combination of hard or semi-hard magnetic material with a soft magnetic material. The hard or semi-hard magnetic material acts as a clamp which holds the soft material in a fixed magnetic state to give rise to a specific response to an interrogating field.
There still remains a need for improvements in this area, and the present invention provides one such solution to these problems.
The present invention relates to a security element which is detected by surveillance systems operating in different frequency ranges. This security element comprises at least two materials of high permeability which, when excited by an external alternating magnetic field, emit each a characteristic signal; the maximum signal components of the two materials lie in different frequency ranges. Further, the signal components of the one material are negligibly low in that particular frequency range in which the signal component of the other material is at a maximum level.
According to an advantageous further feature of the security element of the present invention, the physical properties of the two materials are coordinated such that the component of the characteristic signal of the one material is negligibly low in that particular frequency range in which the signal component of the characteristic signal of the other material is at a maximum level.
Another solution of the present invention provides for the security element to be comprised of a material with Barkhausen effect (high permeability; low coercive force in the frequency range (F1), high coercive force in the frequency range (F2), where F2 greater than  greater than F1), which material is excited by an external magnetic field to emit a characteristic signal detectable in a surveillance system (S1), and of a soft magnetic material (high permeability; low coercive force in the frequency range (F2)) which, when an alternating magnetic field is applied, is excited to emit a characteristic signal detectable in a surveillance system (S2). Further, the physical properties of the two materials are coordinated such that the intensity of the characteristic signal (signal 2) of the soft magnetic material is negligibly low in the detection range (F1) of the surveillance system (S1), while it attains its maximum level in the detection range (F2) of the surveillance system (S2), and that the intensity of the characteristic signal (signal 1) of the material with Barkhausen effect attains a maximum level in the detection range (F1) of the surveillance system (S1), while it is negligibly low in the detection range (F2) of the surveillance system (S2).